The present invention relates to a sheet overlapping detecting method for use particularly in a sheet-fed press.
Conventionally, when sheets (sheets of paper) are to be fed to a sheet-fed press, overlapping of the sheets of paper to be fed is detected.
That is, when a sheet of paper is to be fed from a feeding apparatus to a sheet-fed press (to be referred to as simply a press hereinafter), the leading edge of the sheet of paper is brought into contact with a stopper called a front guide provided at the press-side distal end portion of a feeding table, and then the sheet of paper is fed to the press. In this case, in order to prevent two or more overlapped sheets of paper from being simultaneously fed, a light-emitting device is arranged at the rear surface side of the feeding table in a position close to the front guide, and a through hole is formed in a predetermined portion of the feeding table corresponding to a light-emitting portion of the light-emitting device. In addition, a light-receiving device is arranged at the upper surface side of the feeding table corresponding to the through hole. That is, light emitted from the light-emitting device is radiated in the direction of thickness of a sheet of paper to be fed, and transmission light transmitted through the sheet of paper is received by the light-receiving device and converted into an electrical signal to obtain a received light amount. An output level corresponding to the received light amount is compared with a predetermined determination level, and overlapping of sheets of paper is detected on the basis of the comparison result.
In this case, a light emission amount of light emitted from the light-emitting device and the determination level are preferably set to be optimum values for a corresponding sheet of paper. That is, the relationship between the light emission amount and the output level obtained when the number of sheets of paper is one is different from that obtained when the the number of sheets of paper is two. For example, as shown in FIG. 5, a characteristic curve I is obtained for one sheet of paper, and a characteristic curve II is obtained for two sheets of paper. In this case, the optimum value of the light emission amount is a light emission amount value at which a difference between output levels based on the characteristic curves I and II becomes maximum. The optimum value of the determination level is 1/2 a sum of the output levels based on the characteristic curves I and II obtained at the optimum light emission amount value.
According to a first conventional method, a predetermined determination level is set, and power supply to the light-emitting device is adjusted such that the determination level is positioned at a substantially intermediate point between an output level obtained via the light-receiving device when the number of sheets of paper is one and an output level obtained when the number of sheets of paper is two, thereby setting a light emission amount of light to be emitted from the light-emitting device. According to a second conventional method, predetermined power supply to the light-emitting device is set to determine a light emission amount of light to be emitted from the light-emitting device, and a determination level is set to be positioned at a substantially intermediate point between an output level obtained via the light-receiving device when the number of sheets of paper is one and an output level obtained when the number of sheets of paper is two.
In general, however, a small number of lots of a material is often printed by a press using various types of sheets of paper. That is, since the characteristic curves I and II shown in FIG. 5 change in accordance with the paper quality (including paper thickness, a color, and the like) of paper to be used, the optimum values of a light emission amount and a determination level cannot be kept constant. Therefore, in the above first and second methods, it is difficult to perform stable overlapping detection with high precision for sheets of paper having a wide range of paper quality. In addition, adjustment of the optimum values undesirably largely depends on the skills of an operator.
In addition, in the conventional methods, overlapping of sheets of paper is detected by using a pair of light-emitting and light-receiving devices. Therefore, overlapping detection for sheets of paper having an extremely unstable fiber density (i.e., hungry sheets) is limited by using only the above methods to cause an erroneous operation. For this reason, when overlapping detection is not stably performed, an operator must stop the overlapping detecting function and perform visual inspection, resulting in large physical and mental burdens on the operator.